[AArch64] update "rm" inline asm test (#67472)

Because `x0` is not listed in the clobber list, regalloc could (one day
when #20571 is fixed) allocate `$0` to `x0`:

  ldr x0, x0

This will produce an error when validating the instruction. The intent
of this test FWICT is to check that the parameter in w0 is stored to a
stack slot using w0, since this target triple is the exotic arm64_32
(ILP32). Update the test to simply use "m" constraint. The clobber list
is underconstrained otherwise.